This invention relates to a method and an apparatus for drying veneer and similar products which in the form of a number of sheets or webs are caused to pass through a drying apparatus, especially of the roller type, where the product is dried by means of the flow of hot air and is, during its passage through the apparatus, also exposed to microwave energy radiation which for the purpose of drying underdried zones is applied via transverse ducts.
When the sheets, which are rather thin, a typical thickness being a few millimeters, have left the roller drier they are supplied to a multiple-opening press where, at each press level, a number of sheets are placed on top of each other and glued to form a plywood board. The gluing operation takes place at an elevated temperature.
When the wooden sheets are dried with hot air only they partially get underdried so that they, when leaving the drier, have zones with a high humidity content. Portions comprising such zones must be redried before the sheet can be supplied to the press. The high process temperature in the gluing press means that water trapped in such humid zones is converted to vapour. When a plywood board exits from the press it is no longer subjected to any external pressure keeping the water enclosed. Consequently, the vapour expands so that one or more of the sheet layers in the plywood boards will burst and the board must be rejected. In prior art drying equipment the occurrence frequency of this physical phenomenon is so high that the rejection percentage becomes embarrassingly high. The reason for this is that--in spite of the humidity checking and the redrying above referred to--one cannot safely assume that humid zones will not reach the press.
One object of the present invention is to provide a more complete drying by selective concentration of the microwave energy to the humid zones. This results in a drastic reduction of the rejection percentage.
The veneer sheets are rectangular and produced by subjecting wood logs to a turning operation. The fibres of the wood do then get oriented parallel to the short sides of the rectangle. If the turning tool has cut in such a way that the ends of a fibre are not exposed in the plane of the sheet, the water inside the fibre will be mechanically trapped therein. Consequently, a plurality of such fibres form a humid zone extending transversely to the direction in which the sheet is transported through the drier. The distance between such zones may vary between a few centimeters and several decimeters. It should, however, be observed that also the areas between these zones contain water the presence of which is, however, not equally critical. But under all circumstances a remaining amount of water caused by underdrying is undesired, already for the reason that it means lack of homogenity in the product.
Therefore, a further object of the invention is to make it possible not only to reduce the water content in the veneer sheets exiting from the press but also to distribute the remaining amount of water in the sheet so that, from a humidity point of view, it can be considered homogeneous.